Electrical energy storage systems (EES) are urgently needed by the conventional electricity generation industry. Unlike any other successful commodities markets, the conventional electricity generation industries have little or no storage component. The electricity transmission and distribution systems are operated for the simple one-way transportation from remote and large power plants to consumers. This means that electricity should always be consumed precisely as it is produced. However, the demand for electricity varies considerably emergently, daily and seasonally, and the maximum demand may only last for a few hours each year which leads to inefficient, over-designed and expensive plants. EES allows energy production to be de-coupled from its supply, self generated or purchased. By having large-scale electricity storage capacity available over any time, system planners would need to build only sufficient generating capacity to meet average electrical demand rather than peak demands. This is particularly more important to large utility generation systems, e.g. nuclear power plants, which must operate near full capacity for economic reasons. Therefore, EES can provide substantial benefits including load following, peaking power and standby reserve. Also by providing spinning reserve and a dispatched load, EES can increase the net efficiency of thermal power sources while reducing harmful emissions.
More importantly, EES systems are critically important to intermittent renewable energy supply systems, such as solar photovoltaic, wind turbine and wave power systems. The penetration of renewable resources may displace significant amounts of energy produced by large conventional plant. However, intermittency and non-controllability are inherent characteristics of renewable energy based electricity generation systems, which renders energy supply unstable, even unusable. Such disadvantages have become major hurdles or obstacles to the extensive utility of the renewable energy sources, the green electricity industry. A suitable EES could obviously provide an essential solution to deal with the intermittency of renewable sources and the unpredictability of their output as the surplus could be stored during the periods when intermittent generation exceeds the demand and then be used to cover periods when the load is greater than the generation.
Furthermore, EES is regarded as an imperative technology for the distributed energy resource system (DER) in the near future. Different from the conventional power system which has large, centralized units, DERs are usually installed at the distribution level, close to the places of utilisation, and generate power typically in the small range of a few kW to a few MW. DER is regarded as a sustainable, efficient, reliable and environmentally friendly alternative to the conventional energy system. The energy resource system is undergoing the change to be a mixture of centralized and distributed sub-systems with higher and higher penetration of DERs. However, more drastic load fluctuation and emergent voltage drop are anticipated in DER systems due to smaller capacity and higher possibility of line fault than the conventional power system. EES is identified as a key solution to compensate the power flexibility and provide uninterruptible power supply in case of instantaneous voltage drop for such distributed energy network.
Currently, electrical energy storage technology includes Pumped Hydro, Compressed Air Energy Storage, Secondary Battery, Superconducting Magnetic Energy Storage System, Flywheel, Capacitor and so on. However, only Pumped Hydro and Compressed Air Energy Storage have been commercially utilized currently in a large scale, in consideration of capacity, energy storage duration, energy density, charge-discharge efficiency, life time, operating cost and environmental effects.
Pumped Hydro method pumps water from a lower level reservoir to a higher level converting electrical energy into potential energy during off-peak time, and later the stored water in high level is released through hydro turbine to generate power during periods at high electrical demand. Pumped Hydro is the most widely used electrical energy storage system with advantages of mature technique, high efficiency (70%), large capacity and unlimited energy storage period. However, pumped hydro is under many restrictions for its requiring special geographical conditions to build two reservoirs and dams, its long construction period, huge initial investment, and, what is more important, it gives rise to ecological and immigration problems resulted from the rising water flooding vegetation, even city, owing to the construction of reservoir.
Traditional compressed air energy storage is based on the gas turbine technology. Air is compressed into air-storage vessel with electrical energy converting to potential energy during periods of low power demand (off-peak), and later high pressure air is released, heated by combustor, and expanded through turbine to produce electricity. Compressed-air energy storage system has several advantages: high energy storage capacity, long energy storage period, high efficiency (50%-70%) and comparatively low unit cost. Traditional compressed-air energy storage system, which cannot be used alone, has to be combined only with gas turbine power plant, rather than other types, such as coal-fired power plant, nuclear power plant, wind power plant or solar power plant. As a result, it is not appropriate for the energy strategy of China, which advocates coal-fired power plant rather than oil or gas fired power plant. Furthermore, compressed-air energy storage system still depends on fossil fuel, which not only contaminates the environment with emission of SOx, NOx and carbon dioxide, but also faces the pressure from exhaustion of fossil fuel and increasing price of fossil-based fuel. The most important problem is that large-scale air-storage vessel built for its low energy density requires strict geographical condition, such as rock caves, salt mines and abandoned mines, limiting the range of application of compressed-air energy storage system.
In recent years, much research has been carried out on Compressed-air Energy Storage (CAES), such as Surface Vessel Compressed-air Energy Storage (SVCAES), Advanced Adiabatic Compressed-air Energy Storage (AACAES) and Compressed air storage with humidification (CASH), for eliminating the dependence of CAES on fossil fuel, one of the major problems of CAES. However, the energy density of CAES becomes lower because of nonuse of fossil fuel. It is necessary to find a method to overcome the low energy density and difficult siting of CAES so as to bring out a widely effective use of CAES.